With the widespread use of compact and high-accuracy products such as HDDs (Hard Disk Drives), mobile computers, portable telephones, car telephones, and other mobile communication apparatus in recent years, there has developed an increasing need to further enhance the reliability and accuracy of crystal devices, typically crystal oscillators, used in these products.
The need for increased reliability and increased accuracy has been growing, among others, for crystal gyros used for angular velocity detection in navigation systems or for camera shake control in video cameras.
FIG. 14 is a diagram showing the structure of a prior art tuning-fork type crystal device.
The structure of the prior art tuning-fork type crystal device will be described below (for example, refer to Patent Document 1). The prior art tuning-fork type crystal device 2 comprises: a tuning-fork-shaped crystal plate 11 having a base 2a formed from a crystal and vibrating prongs 2b protruding the base; and electrodes 501 and 511 for generating an electric field across the crystal plate 11. In FIG. 14, the vibrating prongs 2b each refer to the portion from the tip to the first bend of the fork just before the base 2a thereof, and the base 2a refers to the portion beyond the bend.
A prescribed electric field can be produced across the crystal plate 11 by applying different potentials to the respective electrodes 501 and 511. Since the crystal plate 11 is a piezoelectric material, when an electric field is applied across the plate the plate contracts or expands according to the direction of the electric field. As a result, the vibrating prongs 2b of the crystal plate 11 vibrate, and the structure can thus be used as a crystal oscillator 2.
Next, a fabrication method for the prior art tuning-fork type crystal device will be described.
FIG. 16 is a diagram showing part of the fabrication process of the prior art tuning-fork type crystal device. First, as shown in FIG. 16A, a crystal substrate 15 is prepared by forming mask layers 21 and 22 and resist layers 31 and 32 one on top of the other on both the upper and lower surfaces of the substrate. Then, the resist layers 31 and 32 formed on both surfaces of the crystal substrate 15 are exposed to UV radiation of prescribed wavelength through masks 41 of the same mask pattern 41a. The resist layers 31 and 32 are exposed only in portions where the mask pattern 41a is not formed.
Next, the resist layers 31 and 32 are developed as shown in FIG. 16B, forming resist layer patterns 31b and 32b. Here, the resist layer patterns 31b and 32b are each formed in the same shape as the mask pattern 41a of the mask 41.
Further, by using the resist layer patterns 31b and 32b as the masks, the mask layers 21 and 22 are etched as shown in FIG. 16C, forming mask layer patterns 21b and 22b. In this way, the mask layer patterns 21b and 22b are each formed in the same shape as the mask pattern 41a of the mask 41.
Next, the resist layer patterns 31b and 32b are removed as shown in FIG. 16D.
Thereafter, the crystal substrate 15 is etched as shown in FIG. 16E, thus producing the outer shape of the crystal plate 11.
FIG. 17 is a diagram showing the shape of the crystal plate formed by the prior art etching and the shapes of the mask layer patterns for comparison.
As shown in FIG. 17, the major faces 211 and 221 of the crystal plate 11 formed by the etching in FIG. 16E, which were the plane surfaces of the crystal substrate 15, are formed in substantially the same shapes as the respective mask layer patterns 21b and 22b. Since the mask layers 21b and 22b are both substantially the same in shape as the mask pattern 41a, it follows that the two major faces 211 and 221 of the crystal plate 11 are both formed in substantially the same shape.
After the etching in FIG. 16E, the mask layer patterns 21b and 22b are removed, and the electrodes 501 and 511 are formed, completing the fabrication of the tuning-fork type crystal device 2 shown in FIG. 14.
The structure and the fabrication method of the tuning-fork type crystal device according to the prior art described above are used for crystal resonators, crystal oscillators, crystal gyros, and various other applications.
In the prior art tuning-fork type crystal device, it is common practice to form the crystal plate 11 by etching. However, since the crystal has the property that its reaction rate differs depending on the direction it is etched (this property is generally called the anisotropic etching property), a ridge-like unetched portion 111 projecting from a side face is necessarily formed, as shown in FIG. 14, in an intermediate region of the base 2a connecting between the vibrating prongs 2b, that is, in FIG. 14, the portion that originates from the tip of one vibrating prong 2b, passes through the root of the one vibrating prong 2b and through the root of the other vibrating prong 2b, and leads to the tip of that other vibrating prong 2b (this portion is generally referred to as the crotch portion).
When the unetched portion 111 is formed, as shown in FIG. 14, the vibrating prong 2b which should normally vibrate in the X-axis direction vibrates in a direction slightly tilted toward the Z-axis direction as indicated by W1. The vibration W1 produced by tilting toward the Z-axis direction tends to tilt greater toward the Z-axis direction as the unetched portion 111 becomes larger.
FIG. 15 is a diagram showing an enlarged view and a side view of the crotch portion of the tuning fork and its adjacent regions in the prior art tuning-fork type crystal device. Here, FIG. 15(b) shows the shape of the side face as viewed from the direction C in FIG. 15(a). As shown in FIG. 15(b), the unetched portion 111 is formed on the side face of the crystal plate 11 in the shape of a ridge extending obliquely from one major face 211 to the other major face 221 of the crystal plate 11. This produces the same effect as if a prop were placed obliquely across the root portion of the vibrating prong 2b and, with this effect, the direction of vibration is tilted toward the Z-axis direction, producing the vibration W1. In this patent specification, the root refers to the portion at the boundary between the vibrating prong 2b and the base 2a; in FIG. 15(a), the bend corresponds to the root.
Further, as shown in FIG. 15(a), the unetched portion 111 left after the etching is formed only on one vibrating prong side of the crotch portion of the tuning fork, and as a result, the left and right vibrating prongs 2b vibrate in different directions, as shown in FIG. 14.
When the direction of vibration of any one vibrating prong 2b is unstable as described above, the resulting crystal device is often unstable and inaccurate in operation; in the prior art, therefore, the balance between the directions of vibration has been adjusted after forming the crystal plate 11, by performing an additional processing step for appropriately removing portions of the electrodes 501 and 511 and the crystal plate 11.
One proposal has been made in Patent Document 1 to stabilize the vibration of the crystal device. The proposal made in Patent document 1 aims to stabilize the vibration of the crystal device by optimizing the plan shape of the crystal plate 11 according to Patent Document 1, it is stated that the plan shape of one major face 201 is the same as the plan shape of the other major face 211.
However, according to experiments conducted by the present inventors, it has been confirmed that, as long as the plan shape of one major face 201 of the crystal plate 11 is the same as the plan shape of the other major face 211, no appreciable change occurs in the shape of the unetched portion 111 and, in most cases, the unetched portion 111 which causes the unstable vibration is formed in substantially the same shape, though its size may vary somewhat. That is, to whatever shape the mask pattern 41a of the mask 41 alone is optimized that determines the plan shape of the crotch portion of the tuning fork, as shown in Patent Document 1, the unetched portion 111 which causes the unstable vibration is necessarily formed as long as the mask pattern 41a of the same shape is used for both the major faces 201 and 211.
In the case of a crystal gyro as one application example of the tuning-fork type crystal device, the vibration tilted toward the Z axis such as the vibration W1 shown in FIG. 14 becomes a very serious problem. The reason is that, in the crystal gyro, the Z-axis direction is nothing but the direction of the angular velocity to be detected, and any vibration in the Z-axis direction, such as the vibration W1, can affect the accurate detection of the angular velocity. As a result, the prior art crystal gyro has had the problem of low accuracy and low reliability.    Patent Document 1: Japanese Unexamined Patent Publication No. H10-41772 (page 2, FIGS. 2 and 3)